Just the prayer and syllables in question, not the surrounding prayers and other elements of the practice (e.g., the refuge prayer), but this is the prayer that I was thinking of (བླ་མའི་རྣལ་འབྱོར, called the "lamai naljor prayer", also transliterated as "lamé naljor", or translated as "Guru Yoga prayer"):
Chi tsuk dé wa chen pö po drang du
Drin chen tsa wé la ma la söl wa dep
Sang gyé sem su tön pa rin po ché
Rang ngo rang gi shé par jin gyi lop
It has been translated as:
On the crown of my head, in a palace of great bliss
Benevolent root lama, I pray to you.
Precious one that reveals that my own mind is Buddha,
Bless me to recognize my own true nature.
I have also seen it translated a little more poetically:
From the crown of my head, palace of great bliss,
I pray to you, benevolent root lama.
O precious one, grant me the blessings
To recognize my own true nature as a Buddha.
There is a version of this practice found in Opening the Door to Bön by Latri Khenpo Geshe Nyima Dakpa Rinpoche, in the 5th preliminary practice, "Connecting With the Teacher", which provides a virtually identical prayer with a slightly different transliteration and another translation.
It also includes a lot of the surrounding details, visualizations, and prayers. This version of the practice is subtly different from the one that I learned, but the bones are very similar if not identical.
The three syllables are:
- AH – White, Forehead
- OM – Red, Throat
- HUNG – Dark Blue, Mind
These are Bön-specific seed syllables and other branches sometimes use variations.
I don't own the book and so can't speak to its contents in any great detail, but Dilgo Khyentse Rinpoche has written a book called Guru Yoga: According To The Preliminary Practice Of Longchen Nyingtik would seem to cover the practice from a different angle.